1. Field of the Invention
This invention relates to a failure diagnosis device which diagnoses a failure in an exhaust gas recirculation control device.
2. Discussion of Background
Formally, an exhaust gas recirculation control device (hereinafter, EGR control device) which performs a control of exhaust gas recirculation (hereinafter, EGR) as a means of decreasing NO.sub.x in exhaust gas of an internal combustion engine, is widely used. This EGR control device performs the control of EGR by an exhaust pressure control system using BPT (Back Pressure Transducer) valve. A passage area of the EGR valve is controlled by a BPT valve so that an EGR flow quantity becomes a predetermined value. Furthermore, also in a system using VVT (Venturi Vacuum Transducer) and the system in which an EGR control pressure is controlled by a solenoid, similar to the system using the BPT valve, the passage area of the EGR valve is controlled.
As a means of diagnosing a failure in such EGR control device, conventionally, a device described in Japanese Unexamined Patent Publication No. 51746/1987, is proposed. In this device, an intake manifold pressure value wherein an EGR flow quantity exists, and an intake manifold pressure value wherein the EGR flow quantity does not exist, are detected. When the difference between the above pressures is out of a predetermined range, an alarming is generated.
In the conventional failure diagnosis device of an EGR control device, a failure state of the device can not be precisely diagnosed, when the EGR flow quantity is increased due to a deterioration of BPT or the like.